ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 267 (2003) 191–196
Exchange bias effects in ferromagnetic wires
M.K. Husain, A.O. Adeyeye*, C.C. Wang, V. Ng, T.S. Low
Information Storage Materials Laboratory, Department of Electrical and Computer Engineering, National University of Singapore,
4 Engineering Drive 3, Singapore 117576, Singapore
Received 11 February 2003; received in revised form 18 March 2003
Abstract
We have investigated the exchange bias effect in micron-sized ferromagnetic wires made from Co and Ni80Fe20 films.
The wires were fabricated using optical lithography, metallization by sputtering and lift-off technique. Magnetotransport measurements were performed at temperatures ranging from 3 to 300 K. We observed marked changes in the
magnetoresistance (MR) properties as the temperature is varied. At 300 K, the field at which the sharp peak occurs
corresponding to the magnetization reversal of the Co wires is 167 Oe and is symmetrical about the origin. As the
temperature was decreased to 3 K, we observed a shift in the peak positions of the MR characteristics for both
the forward and reverse field sweeps corresponding to a loop shift of 582 Oe in the field axis. The asymmetric shift in the
MR loops at low temperatures clearly indicates the exchange bias between ferromagnetic (Co) and antiferromagnetic
parts (Co-oxide at the surfaces) from natural oxidation. Ni80Fe20 wires of the same geometry showed similar effect with
a low exchange bias field. The onset of exchange biasing effect is found to be 70 and 15 K for the Co and Ni80Fe20 wires,
respectively. A striking effect is the existence of exchange biasing effect from the sidewalls of the wires even when the
wires were capped with Au film.
r 2003 Elsevier Science B.V. All rights reserved.
Keywords: Exchange bias; Magnetic wires; Magnetoresistance
1. Introduction
Laterally defined micron and sub-micron magnetic structures have received great attention
because of the advances in lithographic and
magnetic measurement techniques [1,2] as well as
due to their potential for practical applications, for
example, data storage devices [3]. Dramatic
changes in magnetic properties such as coercivity,
saturation that occur due to the change in lateral
*Corresponding author. Tel.: +65-68745071; fax: +6567791103.
E-mail address: eleaao@nus.edu.sg (A.O. Adeyeye).
dimensions provide the main motivation for
studying these devices. A detail review of the
general effects of patterning in ferromagnetic films
can be found in Ref. [3] and references therein.
One of the main properties that is of interest in
magnetic ordered structures is the modification of
magnetization reversal processes at low temperatures. One such phenomenon, ‘‘exchange bias’’
between ferromagnetic and antiferromagnetic materials, was first reported in ultrafine particles of
Co with a cobaltous oxide shell [4]. This biasing
effect was also observed in many different systems
containing ferromagnetic (FM)–antiferromagnetic
(AFM) interfaces [5–10]. The exchange bias
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0304-8853(03)00352-4
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properties of oxidized transition metal nanoparticles and films have been extensively studied [4,
11–13]. In an MR head the output response is not
linear. Therefore an exchange bias is required for
linearization, which can be achieved by coupling
FM to an AFM [14]. Research on exchange
biasing effect in ferromagnetic wires is therefore
of interest because of its potential application in
the information storage. Earlier works on magnetic wires were mostly concentrated on size
dependence and orientation dependence on field
[2,15,16]. Fraune et al. [17] have investigated the
size dependence of exchange bias in NiO/Ni
nanostructures. They observed that for NiO/Ni
wires narrower than 3 mm, the exchange bias field
significantly depends on the wire width. In another
experiment, Otani et al. [18] have investigated the
magnetization reversal processes of the Fe19Ni81/
NiO sub-micron exchange coupled bilayer wires.
They observed a size effect on the exchange
coupling field when the wire is finer than 1 mm.
Recently, exchange bias effect in patterned FeMn/
NiFe bilayer wire array have been investigated by
Guo et al. [19].
In the present work, we have studied the
magnetization reversal of array of lithographically
defined micron sized Co and Ni80Fe20 wires at
temperatures ranging from 3 to 300 K. As we
reduced the temperature beyond a critical point,
we observed a shift in the peak positions of the
MR characteristics for both the forward and
reverse field sweeps. This asymmetric shift in the
MR loops at low temperatures clearly indicates the
exchange bias between the ferromagnetic and
antiferromagnetic components. The latter appears
due to the formation of antiferromagnetic Cooxides and Ni80Fe20 oxides at the surfaces of the
corresponding FM wires during fabrication.
deposition were set at 10 mTorr and 10 standard
cm3/min (sccm), respectively, and a DC power of
100 W was used. The wires are each of width
w ¼ 2:5 mm, spacing s ¼ 5 mm, thickness t ¼
40 nm, length l ¼ 4 mm and extend over a distance
of 4 mm. Electrical contacts were made using a
combination of standard optical lithography,
metallization and lift-off of 150 nm Al. The
contacts were made parallel to the wire axis. A
scanning electron micrograph of the array of Co
(40 nm) wires is shown in Fig. 1. For MR
measurements a DC sense current of 1 mA was
passed through the current leads and the resistance
was recorded automatically using a four terminal
method as the in plane magnetic field was swept.
The field was applied perpendicular to the wire
axis. The MR measurements were performed at
temperatures ranging from 3 to 300 K. The low
temperature measurements were performed using
a Janis Model SVT Research Cryostat that
enabled a temperature variation from 1.5 K to
room temperature.
2. Experimental procedures
3. Results and discussion
The magnetic wires were fabricated on Si (0 0 1)
substrates using the optical lithography, metallization and lift-off techniques. A base pressure of
3 107 Torr was maintained during sputtering of
the magnetic materials at room temperature. The
working pressure and Ar gas flow rate during film
Fig. 2 shows the MR curves for 40 nm thick Co
wires when the applied field is perpendicular to the
wire long axis for various temperatures. First, a
negative field sufficiently large to saturate the wires
was applied perpendicular to the wire axis. As the
magnetic field was swept back toward a positive
y
30µm
x
Fig. 1. Scanning electron micrograph of Co wires with width,
w ¼ 2:5 mm and spacing, s ¼ 5 mm.
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M.K. Husain et al. / Journal of Magnetism and Magnetic Materials 267 (2003) 191–196
Hc1
H c2
0.04
0
-0.3
0.3
T=300 K
193
Table 1
The peak positions on the MR curves of Co wires at various
temperatures
300 K
50 K
3K
0.02
MR (%)
0
-1
-0.5
0
0.1
0.5
-2
0
2
0.05
-7
-3.5
0
0.1
3.5
-2
7
0
2
T=3 K
0.05
0
-7
-3.5
0
3.5
Hc2
Hc1
Hc2
Hc1
Hc2
167 Oe
167 Oe
1250 Oe
638 Oe
1400 Oe
236 Oe
1
T= 50 K
0
Hc1
7
Applied Field (KOe)
Fig. 2. MR curves of Co wire array for various temperatures
with field applied perpendicular to the wire axis. The low field
region is magnified in the inset.
value at constant rate, the MR increased until it
reached the peak (Hc1 ) when the magnetization of
the ferromagnetic wires switched its direction and
tend to align with the field. In a similar fashion, the
peak (Hc2 ) on the MR curve was obtained after
applying a large positive field, and then sweeping
the field towards negative values. The crossover at
large negative fields could be due to a slight drift in
temperature during measurement. The peak positions (Hc1 and Hc2 ) shown in Fig. 2 in the MR
curves is due to AMR effect corresponding to the
switching of magnetization in Co wires. We
observed that the peak positions shift noticeably
with temperature.
Table 1 shows the field values at which the peaks
(Hc1 and Hc2 ) occur in the MR characteristics for
Co wires at different temperatures. At 300 K the
peaks are symmetrical around the origin with a
switching field 167 Oe. As the temperature is
decreased we observed an unusual asymmetry in
the peak positions for the forward and backward
field sweeps. The peaks in the MR loops shifts
asymmetrically to higher positive fields. The
asymmetric shift in the peak positions of the MR
loops at low temperatures indicates the appearance of exchange bias (HE ) in the system. The
biasing occurred due to the coupling of spins in the
FM and AFM when cooled below Ne! el temperature (TN ). HE depends strongly on the spin
structure at the interface [8–10].
Shifting of the fields to higher fields corresponds
to the increase of coercivity with decreasing
temperature. The overall coercivity is defined as
HC ¼ 12jHc1 Hc2 j;
ð1Þ
where Hc1 and Hc2 are the coercive fields
corresponding to the right and left of the MR
loops.
The asymmetric shift in peak positions is best
described in terms of an effective internal ‘‘exchange field’’ HE , which arises from the FM-AFM
interface, is defined as [12]
HE ¼ 12jHc1 þ Hc2 j:
ð2Þ
For Co wires, when the temperature is 50 K, an
exchange bias field of 306 Oe is measured. For
T ¼ 3 K, we measured a bias field of 582 Oe.
Similar measurements were performed on
Ni80Fe20 wires having the same geometry and
fabricated under similar conditions as Co wires.
Representative MR curves at various temperatures
are shown in Fig. 3. We also observed an
asymmetric shift in peaks in the MR characteristics at low temperatures. However, the magnitude of the shift is small when compared with that
observed in Co wires of the same lateral dimensions. During the fabrication process, the ferromagnetic wires were exposed to the atmosphere
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194
600
0
-0.1
0.1
T=300 K
0.05
MR (%)
0
HE (Oe)
Co
Ni Fe
0.1
80
400
20
200
0
-1
-0.5
0
0.5
0.4
1
0
-0.5
0.5
T= 15 K
0
20
40
60
80
T (K)
100
120
140
Fig. 4. Exchange bias field HE vs. temperature for Co and
Ni80Fe20 wires.
0.2
0
-1
-0.5
0
0.4
0.5
1
0
-0.5
0.5
T=3 K
0.2
0
-1
-0.5
0
0.5
1
Applied Field (KOe)
Fig. 3. MR curves of Ni80Fe20 wire array for various
temperatures with field applied perpendicular to the wire axis.
The low field region is magnified in the inset.
before measurements resulting in the natural
oxidation of the wires top surface and sidewalls.
We also observed in Fig. 3 (for To15 K) that
the forward and reverse field loops are asymmetric
in shape and the peak height is different. This
asymmetry is not an artifact of sample misalignment, or the current direction misalignment as the
asymmetry disappears when the sample is no
longer exchange biased at 300 K. This effect has
been observed by Leighton et al. [20] and can be
attributed to the asymmetrical magnetization
reversal.
In order to determine the onset of exchange bias
effect in both Co and Ni80Fe20 wires we have
performed a systematic study on the asymmetric
shift in the peaks as a function of temperature.
Shown in Fig. 4 is the temperature dependence of
the exchange bias field for both the Co and
Ni80Fe20 ferromagnetic wires. For To100 K we
observed that the peak positions are symmetric
about the origin. As the temperature was reduced
further to 70 K a marked shift in the peak position
was observed for Co wires. From the asymmetric
shift of the peak positions we were able to
determine the exchange bias field of 133 Oe.
Further decrease in temperature resulted in a
marked exchange bias field. The blocking temperature (TB ) of the antiferromagnetic Co-oxide at
the wire edges estimated from Fig. 4 is B70 K. We
have measured the atomic composition of the
oxide on the top surface using X-ray photoelectron
spectroscopy (XPS). We observed the existence of
CoO and Co3O4 on the surface. Therefore,
possible antiferromagnetic components in our
sample could be CoO and Co3O4. Bulk Co3O4
and CoO have TN of about 30 K [21] and 290 K
[13,22], respectively. It has been observed by Lund
et al. [23] that TB depends strongly on the
thickness and anisotropy of the AFM. Therefore
the large difference between TB and TN of Cooxides in our sample may be due to low thickness
or low anisotropy (because of defects) of the
AFM. There might also be some contribution
from other AFM phases [7,10].
For Ni80Fe20 wires, however, exchange-biasing
fields of 48 and 128 Oe were measured when the
temperature is reduced from 15 to 3 K, respectively. This is quite in agreement with similar work
done by O’Grady et al. [24] on NiFe layers and
NiFe/Cr multilayers. The possible candidates for
AFM in Ni80Fe20 are the oxides of Fe or Ni. This,
however, could not be readily established from our
XPS measurements. The Ne! el temperatures (TN )
of NiO and FeO are 523 and 198 K, respectively
and therefore the most likely component is FeO.
Nevertheless, it is clear that only ferromagnetic
parts contribute to magnetization at 20 K. This is
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1000
Co
Ni Fe
HC (Oe)
800
80
20
600
400
200
0
0
20
40
60
80
100
120
140
T (K)
Fig. 5. Coercivity HC vs. temperature for Co and Ni80Fe20
wires.
MR (%)
0.45
0.3
0.15
0
-3.5
0
3.5
Field (KOe)
Fig. 6. MR curve at T ¼ 3 K for Co (40 nm)/Au (5 nm) wires
with field applied perpendicular to wire axis.
HE (Oe)
25
20
15
10
5
0
-5
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
340
HC (Oe)
consistent as the exchange bias disappears above
20 K.
Shown in Fig. 5 is a plot of the coercivity (HC )
measured from the peak positions as a function of
temperature (T) for both Co and Ni80Fe20 wires.
We observed a marked increase in HC as T is
decreased. This increase in HC below TN can be
intuitively explained. When the FM spin switches
its direction it irreversibly drags the AFM spins
thus increasing the FM coercivity. However, direct
dependence on temperature is difficult to understand because HC is also affected by the microstructures of FM layer that changes from system
to system [10,25,26].
Ni80Fe20 wires show an unusual effect when we
compare HC (TB B50 K) and HE curves of Figs. 5
and 4, respectively. The difference in TB may be
due to coercivity enhancement above TB as
observed by Leighton et al. [27] which is attributed
to the spin fluctuations in the AFM layer inducing
additional anisotropy in the FM layer.
In order to investigate the contribution of the
wire sidewalls to the exchange bias field, we
fabricated an array of Co wires of the same lateral
dimensions and under the same fabrication conditions, except that we capped the wires top surface
with 5 nm of Au to prevent oxidation. The
sidewalls of the wires were allowed to oxidize
naturally. Fig. 6 shows corresponding MR response at 3 K. We observed that the asymmetric
shift exists in the peaks in the MR loop though it is
very small compared to the one in Fig. 2. The
exchange bias field at 3 K is 22 Oe compared with
an exchange bias field of 582 Oe for the sample
without Au capping. While the exchange biasing
195
330
320
310
300
290
T(K)
Fig. 7. HE ; HC vs. temperature for Co wire array capped with
Au (5 nm).
effect from the top surface of the wires observed in
Fig. 2 may not be new, the effect found in capped
wires shown in Fig. 6 is very striking and
surprising.
HC and HE is plotted in Fig. 7 as a function of
temperature for the Au capped Co wire array. TB
is found to be 30 K which is much lower than what
was observed in the uncapped wires. HC values at
different temperatures are also observed to be
much lower in capped wires. This can be attributed
to the comparatively less effective AFM spins in
the capped wires due to the absence of contribution of exchange bias from the top surface of the
wires.
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4. Conclusion
We have investigated the temperature-dependent MR properties of arrays of micron-sized Co
and Ni80Fe20 wires. We observed an exchange
biasing effect at low temperatures manifested by
an asymmetric shift in the peak positions on their
MR characteristics. We have attributed this to the
presence of antiferromagnetic material at the
surfaces of the wires due to the natural oxidation
of the magnetic materials during fabrication
process. The onset of exchange biasing effect is
found to be 70 and 15 K for the Co wires and
Ni80Fe20 wires, respectively. The exchange biasing
effect was observed even when the wires top
surfaces were capped with Au.
Acknowledgements
This work was supported by the National
University of Singapore (NUS) under grant
R263-000-180-112. M.K.H. would like to thank
NUS for their Research Scholarship.
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